If you’re pregnant—or trying to conceive—the Zika virus is probably a top-of-mind concern right now, and with good reason: This mosquito-borne virus is dominating headlines with its scary multi-country advance and potentially devastating consequences for pregnant women and their babies.

Zika surfaced just over a year ago in South America, and Brazil has been disproportionately affected, with thousands of babies suffering severe birth defects, including brain damage, in utero when their mothers contracted the virus. But it has now spread to more than three dozen countries and territories in the Americas, and has recently landed in the United States (although it’s important to note that these U.S. cases were brought by returning travelers from affected regions). According to the Centers for Disease Control and Prevention (CDC), 168 pregnant women in the US and the District of Columbia have been diagnosed with Zika and another 142 have been identified in the US territories, which includes the US Virgin Islands and Puerto Rico.

Per the CDC, mosquitoes in the continental United States or Hawaii have not spread Zika. However, lab tests have confirmed Zika virus in travelers returning to the United States. These travelers have gotten the virus from mosquito bites and some non-travelers got Zika through sex with a traveler. Cases of local transmission have been confirmed in three US territories: Puerto Rico, the US Virgin Islands, and American Samoa.

The virus is likely to spread further, according to the World Health Organization (WHO), because the mosquito that transmits Zika is in all but two countries of the Americas, and the people in these regions lack immunity to the virus.

If you’re expecting (and frankly, even if you’re not), it’s crucial to arm yourself with information and up-to-date advice. This is what you need to know:

What is Zika virus?

The Zika virus is an insect-borne illness that can be primarily transmitted by infected Aedes mosquitoes, the same kind that carry dengue and yellow fever. The name comes from the Zika Forest in Uganda where monkeys with the virus were first found in 1947.

Why is it dangerous?

For the relatively few people who show signs of a Zika infection, the illness is often very mild. But in pregnant woman, the effects can be devastating, and can include pregnancy loss or a baby born with an abnormally small head and brain—a condition known as microcephaly, says Edward R.B. McCabe, M.D., Ph.D., Senior Vice President and Chief Medical Officer of the March of Dimes. Microcephaly may be associated with developmental delays, mental retardation, and seizures, and in some cases can be fatal.

Until recently, Zika virus had only been associated with significant risk to the fetus—it wasn’t established that the effects were actually caused by it. But now the news has changed and health officials can report a direct link between Zika and microcephaly. Still, there are many unknowns—including how likely it is that an infection in a pregnant woman will be passed on to her fetus; whether some fetuses are infected but don’t develop microcephaly; how often pregnancy loss may occur in expecting women with Zika virus; and whether pregnancy makes women more susceptible to the virus, says MarjorieTreadwell, M.D., director of the Fetal Diagnostic Center at the University of Michigan and a maternal and fetal medicine expert.

To date, there have been no infants born with microcephaly and other poor outcomes linked to locally acquired Zika virus infection during pregnancy in the continental United States. One infant with microcephaly linked to travel-associated Zika virus infection during pregnancy has been reported in Hawaii as well as one with microcephaly born in a hospital in New Jersey to a woman who had previously tested positive for Zika virus infection and had traveled to Central America during pregnancy.

While the Zika virus remains in the blood of an infected person for a few days to a week, according to the CDC, there’s no current evidence to suggest that it poses a risk of birth defects in future pregnancies. And Zika won’t cause infections in a baby that’s conceived after the virus has left the bloodstream.